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Compressed alkanes in reversible encapsulation complexes

Nature Chemistry volume 1, pages 87–90 (2009)Cite this article

Abstract

Simple alkanes feature fully extended conformations as their lowest-energy shapes but can assume coiled, compressed conformations in small spaces. A series of normal alkanes, C16 to C19, were encapsulated in self-assembled, hydrogen-bonded complexes. Coiling of the longer alkanes was observed by NMR spectroscopy. The coiling exerts pressure on the interior; the hydrogen bonding seams are loosened, and rotation of the capsule's components occurs on the NMR timescale. The rotation results in interconversion of mirror-image capsule assemblies (racemization). The racemization rates were determined and shown to increase with the length of the alkane, the longer alkanes exerting more pressure. Free energies of activation for racemization were determined at the coalescence temperatures, and were ΔG‡ = 15.7, 16.7 and 17.2 kcal mol−1 for C19, C18 and C17, respectively. The shorter C16 was encapsulated in its fully extended conformation, and does not seem to exert pressure inside the capsule.

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Figure 1: Encapsulation of coiled alkanes in a cylindrical capsule.
Figure 2: Insertion of spacer elements in a dimeric capsule creates an extended, chiral capsule.
Figure 3: Coiled alkanes in an extended capsule create internal pressure.
Figure 4: Racemization of the extended capsule.

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Acknowledgements

We are grateful to the Skaggs Institute for support.

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Authors and Affiliations

  1. The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, 92037, California, USA

    Dariush Ajami & Julius Rebek Jr

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  1. Dariush Ajami
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Correspondence to Julius Rebek Jr.

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Ajami, D., Rebek, J. Compressed alkanes in reversible encapsulation complexes. Nature Chem 1, 87–90 (2009). https://doi.org/10.1038/nchem.111

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